This invention relates generally to a cervical spinal implant assembly for holding adjacent vertebral bones fixed, and more particularly to a cervical plate assembly having polyaxial screws which seat into sliding and locking socket components.
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease are a few of the causes which can result in spinal pathologies for which permanent immobilization of multiple vertebrae may be necessary. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies.
The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. Posterior fixation is much more commonly used in the lower back, i.e., the sacral, lumbar, and lower thoracic regions, than in the upper regions of the thoracic and the cervical spine. The use of screw and plate assemblies for stabilization and immobilization via lateral or anterior entrance in these upper regions is, however, common.
Because the cervical spine is routinely subject to mechanical loads which cycle during movement, one of the primary concerns of physicians performing cervical plate implantation surgeries, as well as of the patients in whom the implants are placed, is the risk of screw pullout. This is of particular concern in the cervical region because of the critical vessels which abut the anterior surfaces of the cervical spine. Screw pull-out occurs when the cylindrical portion of the bone which surrounds the inserted screw fails. A bone screw which is implanted perpendicular to the plate is particularly weak because the region of the bone which must fail for pull-out to occur is only as large as the outer diameter of the screw threads. It has been found that for pull-out to occur for a pair of screws which are angled inward, xe2x80x9ctoe nailedxe2x80x9d, or ones which diverge within the bone, the amount of bone which must fail increases substantially as compared to pairs of screws which are implanted in parallel along the axis that the loading force is applied. It has, therefore, been an object of those in the art to provide a screw plate assembly which permits the screws to be entered into the vertebral body at angles other than 90 degrees.
As mentioned above, a great concern with screws being implanted in the anterior portion spine, most particularly in the cervical spine, is that their are important internal tissue structures which, because of their proximity to the implant, may be damaged by a dislocated screw. In the cervical spine, the esophagus is located directly in front of the anterior surface of the vertebral body, and therefore, in potential contact with an implanted cervical plate. Breaches of the esophageal wall permit bacterial contamination of the surrounding tissues, including the critical nerves in and around the spinal cord. Such contamination can be fatal. Because screw pull-out represents one of the largest risks of esophageal perforation, it has been an object of those in the art to produce a cervical screw plate design having a locking means which couples, not only the plate to the bone, but locks the screw to the plate. In such a design, it is intended that, even if the bone holding the screw fails, the screw will not separate from the plate.
One screw plate design which has been offered to provide physicians and patients with a reduced risk of pull-out or damage to proximal tissues is the Orion (Reg. Trademark) Anterior Cervical Plate System of Sofamor Danek USA, 1800 Pyramid Place, Memphis, Tenn. 38132. The Orion(trademark) system teaches a plate having two pair of guide holes through which the screws are inserted to fix the plate to the vertebral body. The plate further includes external annular recessions about each of the guide holes which are radially non-symmetric in depth. More particularly, the annular recessions serve as specific angle guides for the screws so that they may be inserted non-perpendicularly with respect to the overall curvature of the plate. In addition, the Orion(trademark) plate includes an additional threaded hole disposed between each of the pairs of guide holes so that a corresponding set screw may be inserted to lock the bone screws to the plate.
Although the Orion(trademark) system achieved certain advantages over prior cervical screw plate assemblies, it is not without substantial drawbacks. Specifically, a given plate can accommodate only one screw-in angulation per hole, preferably in accordance with the angle of the annular recession. This is undesirable, in that physicians often must inspect the vertebral bodies during the implantation procedure before making the decision as to which screw-in angle is the ideal. By forcing the physician to chose from a limited set of angles, it is unavoidable that physicians will be forced to implant plates having screws which were positioned non-ideally. While providing a variety of plates having different angle guide holes and annular recession orientations is possible, the complexity and expense of providing a full spectrum of plates available in the operating room for the surgeon to choose from is undesirable. It is a failure of the system that one plate cannot accommodate a variety of different screw-in angles.
It is further a failure of the Orion(trademark) system that an extra set screw is required to lock the screw to the plate. Plates for use in the cervical spine are very thin, and if the screw head already rests in an annular recess, and there is to be enough room for the head of the set screw to rest on top of the head of the bone screw, the thickness of the remaining plate must be reduced even further. The thinner the plate is at the load bearing pointsxe2x80x94the guide holesxe2x80x94the weaker the plate is overall.
Another critical failure of the Orion(trademark) plate, is its inability to permit variation in the screw insertion point relative to the plate and other screws. More particularly, it is often desirable for the surgeon to be able to insert a screw at a slightly different point on the cervical one on one lateral side as compared with the opposite lateral side. With the Orion(trademark) plate, a surgeon would have to offset the entire plate in order to accommodate this relative screw positioning offset. It would therefore be a significant advantage to provide a plate which permits relative screw offset without having to offset the plate itself.
While the preceding discussion has focused on a specific cervical screw plate system and its failures, the same failures apply to the art of lumbar and thoracic immobilizing screw plate systems which are presently available as well. It is therefore, an object of the present invention to provide a new and novel cervical, thoracic, and/or lumbar screw plate design having a polyaxial coupling of the screw to the plate, whereby a single plate is compatible with a wide range of screw-in angles.
It is also an object of the present invention to provide an orthopedic screw plate assembly which has a simple and effective locking mechanism for locking the bone screw to the plate.
It is still further an object of the present invention to provide a screw plate assembly having a retaining means for preventing screw pull-out in the event of a failure of the locking mechanism.
It is also an important object of this invention to provide a screw and plate assembly which can accommodate a variety of different screw insertion points without requiring an offset of the plate itself.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter. into the tapered through hole. As the tapered surface of the coupling element advances, the lateral constraining forces of the mutual tapers (of the coupling element and the through hole) causes the element to contract slightly as the axial slot or slots are narrowed. This contraction causes the interior volume to crush-lock to the semi-spherical head of the screw thereby locking it at the given angulation and to the plate.